The invention relates to a rotary drill bit for drilling oil and gas wells and, more particularly, to a drill bit suitable for up to hard formations having cutting preforms of polycrystalline diamonds on tungsten carbide substrates.
In rotary drilling, the bit is fixed on the end of a rotating drill pipe inside a casing, the drill pipe being lowered as the drilling progresses. A heavy artificial substance known as drilling mud is circulated down through the drill pipe, out through the bit and back up the casing to remove rock fragments. The drilling mud cools the bit, washes the cutting elements so they present a clean cutting face whereby the cutting takes place and, as indicated, lifts or carries debris resulting from the drilling to the surface. For the drilling mud to carry out these functions, it is necessary its velocity through the bit's fluid entrances and channels be high without causing an undesirably high back pressure so it moves quickly across the face and is discharged rapidly and efficiently up the junk slots. It is important clogging of the bit be prevented with rapid removal of cuttings and also that undue stress be avoided.
Rapid penetration of the earth and longer runs improve efficiency of the drilling operation very substantially. Preform cutting elements as disclosed in U.S. Pat. Nos. 3,745,623, 3,767,371, 4,109,737 and 4,156,329 have been utilized by the inventor and others to provide improved penetration in soft to medium-hard formations such as salt, shale, anhydrite, carbonates, marls, clay and sandstones. But when drilling in harder formations, difficulties have been encountered. Solutions to such problems have been suggested as disclosed, for example, in U.S. Pat. No. 4,006,788 to L. Garner, which applies to a rotary drill bit provided with rolling cone cutters. The patent spells out the need for rotary rock bits which work well in various types of formations, soft or hard, encountered in deep wells where it is highly desirable to penetrate long distances before changing the bits required.